Just tested the krita-lazybrush-kazakov branch:

This is applicable to both artists and animators, both need a quick way to do flat colours quickly and to produce masks so they can keep their painting within the lines. There's only one feature that's different for animators, and that's the ability to colour across multiple frames in onion-skin mode.

I think marking frames with arbitrary colours is a good way to go about it, as we should probably stay unopinionated on what exactly it is the user wants to label. There's already a point of contention since the typical traditional animation process also adds a breakdown/extremes in addition to the keyframes and inbetweens.